British Prime Minister Theresa May signed the letter on Tuesday, a photo released by her office showed.

Officials declined to say how the letter was being conveyed to Brussels, citing security reasons.

May is due to formally announce that she has triggered the Brexit process to parliament at 1130 GMT.

"We are one great union of people and nations with a proud history and a bright future. And, now that the decision has been made to leave the EU, it is time to come together," she will say, according to the speech published in advance by Downing Street.